Group III intron is a class of introns found in mRNA genes of chloroplasts in euglenid protists. They have a conventional group II-type dVI with a bulged adenosine, a streamlined dI, no dII-dV, and a relaxed splice site consensus. Splicing is by two transesterification reactions with a dVI bulged adenosine as initiating nucleophile; the intron is excised as a lariat.
Discovery and identification
Montandon,P. and Stutz,E. (1984) and Hallick,R.B. et al. (1988 and 1989) reported examples of a novel type of introns in Euglena chloroplast. In 1989, David A.Christopher and Richard B.Hallick proposed the title, Group III introns to identify this new class with the following characteristics:
In 1994, discovery of a group III intron with a length of one order of magnitude longer indicated that length alone is not the determinant of splicing in Group III introns (Copertino DW., Hall ET. et al. 1994)
Splicing of group III introns occurs through lariat and circular RNA formation. Similarities between group III and nuclear introns include conserved 5' boundary sequences, lariat formation, lack of internal structure, and ability to use alternate splice boundaries.